1. Technical Field
The present disclosure relates to the preparation of samples for electron microscopes.
2. Description of the Related Art
Electron microscopy provides significantly higher resolution and greater depth of focus than optical microscopy.
In a scanning electron microscope (also referred to as “SEM”), a primary electron beam is focused to a fine spot that scans the surface to be observed. Secondary electrons are emitted from the surface as it is impacted by the primary beam. The secondary electrons are detected, and an image is formed, with the brightness at each point of the image being determined by the number of secondary electrons detected when the beam impacts a corresponding spot on the surface.
In a transmission electron microscope (also referred to as “TEM”), a broad beam impacts the sample and electrons that are transmitted through the sample are focused to form an image of the sample. The sample is sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite site.
Samples typically are pre-processed (generally referred to as “sample preparation”) to be suitably viewed under an electron microscope. Sample preparation strongly depends on the sample type and the desired analysis.
Specifically, if the sample is or includes an electronic semiconductor device, such as for example an electronic circuit integrated in a chip made in a semiconductor material (e.g., silicon), the sample preparation may provide for the execution of target surfacing operations, followed by a ion beam milling operation. The target surfacing operations exploit mechanical tools for milling, sawing, drilling, grounding and/or polishing the semiconductor device, while the ion beam milling operation uses high energy ion bombardment to remove material or modify the surface of the semiconductor device. These operations may be performed directly on the chip as such (naked chip) or with the chip embedded in a package.
The target surfacing operations and the ion beam milling operation are typically carried out by means of two different apparatuses. For example, the target surfacing operations may be carried out by a target surfacing apparatus such as the Leica EM TXP®, while the ion beam milling operations by a ion beam milling apparatus such as the Leica EM RES101®.
According to a solution known in the art, a sample preparation of a semiconductor device provides the following operations.
Firstly, as illustrated in FIG. 1A, the semiconductor device, identified with the reference 100, is inserted in the target surfacing apparatus for being subjected to targeting surfacing operations.
In order to insert the semiconductor device 100 in the target surfacing apparatus, the semiconductor device 100 is coupled with a specimen holder. The specimen holder comprises a stub member 110 including a substantially flat support element 112 with a face centrally provided with a protruding pin element 114. A clamp element 120 adapted to hold the semiconductor device 100 is provided on the other face of the support element 112. The clamp element 120 comprises in turn two faced jaw members 122, 124, each one provided with a respective threaded hole 126 rotatably housing a corresponding set screw (not illustrated in the figures). In an open configuration, the clamp element 120 has the two faced jaw members 122, 124 which are spaced apart from each other. The clamp element 120 is brought to a closed configuration by tightening the set screws so as to force the jaw members 122, 124 toward each others. The semiconductor 100 is inserted within the free space between the two jaw members 122, 124 when the clamp element 120 is in the open configuration, and then the set screws are tightened to bring the clamp element 120 in the closed configuration so that the semiconductor device 100 is held between the jaw members 122, 124 through the application of inward pressure.
The stub member 110 is then coupled—e.g., through a proper adapter member 130—to a pivot arm 140 of the target surfacing apparatus. The adapter member 140 comprises a further clamp element 132 adapted to be opened/closed by tightening/loosing a set screw (not illustrated in the figures) rotatably housed in a threaded hole 134, and a protruding pin element 136. The stub member 110 is coupled with the adapter member 130 by clamping the pin element 114 of the former in the clamp element 132 of the latter. The stub-adapter assembly is then coupled to the pivot arm 140 of the target surfacing apparatus by inserting the protruding pin element 136 of the adapter member 130 into a corresponding opening 145 of the pivot arm 140.
As illustrated in FIG. 1B, the semiconductor device 100 is then processed by exploiting one or more mechanical tools 150 of the target surfacing apparatus, such as for example rotating diamond and tungsten carbide millers, diamond disc cutters and/or lapping inserts. When the processing operations carried out by means of the mechanical tools 150 are ended, the stub member 110 is removed from the pivot arm 140, and then the semiconductor device 100 is released from the clamp element 120.
Afterward, as illustrated in FIG. 2A, the semiconductor device 100 is inserted in the ion beam milling apparatus for being subjected to ion beam milling operations.
For this purpose, the semiconductor device 100 is coupled with a new specimen holder including a stub member 210 substantially similar to the stub member 110. The stub member 210 includes a substantially flat support element 212 having a face centrally provided with a protruding pin element 214. A clamp element 220 adapted to hold the semiconductor device 100 is provided on the other face of the support element 212. The clamp element 220 comprises in turn two faced jaw members 222, 224, each one provided with a respective threaded hole 226 rotatably housing a corresponding set screw (not illustrated in the figures). The semiconductor 100 is inserted within the free space between the two jaw members 222, 224 when the clamp element 220 is in the open configuration, and then the set screws are tightened to bring the clamp element 220 in the closed configuration so that the semiconductor device 100 is held between the jaw members 222, 224 through the application of inward pressure.
The stub member 210 is then coupled—e.g., through a proper adapter member 230—in a working support member 240 designed to be housed in the ion beam milling apparatus. The adapter member 230 comprises an upper support portion 232 provided with a hole 234 for receiving the pin element 214 of the stub member 210, and a protruding engaging element 236. A threaded hole 238 is provided on the support portion 232 for rotatably housing a set screw (not illustrated in figure) adapted to fasten the stub member 210 to the adapter member 230 when the pin element 214 is inserted in the hole 234.
The working support member 240 comprises an hollow cylindrical base element 242 whose top edge is surrounded by a handling ring element 244 that protrudes transversally to the side surface of the base element 242. An opening 246 crosses the base element 242 from the top to the bottom edges thereof. The stub-adapter assembly is coupled with the working support member 240 by fitting the engaging element 236 of the former into the opening 246 of the latter. A threaded hole 248 is provided on the side surface of the base element 242 for housing a set screw (not illustrated) which is adapted to be tightened for fastening the stub-adapter assembly to the working support member 240 when the engaging element 236 is fitted in the opening 246.
In a calibration phase directed to set the working distance for the subsequent ion beam milling operation, the stub-adapter assembly is fitted in the working support member 240 with the set screw housed in the hole 248 that is loose, in such a way that the engaging element 236 is completely inserted in the opening 246; then, by grabbing the handling ring element 244, the stub-adapter-support is inserted in a calibration apparatus (not illustrated) having an arrangement for pushing upwards the engaging element 236 (and thus, the stub-adapter assembly) within the opening 246 until the stub-adapter assembly is displaced with respect to the support member 240 by a desired working distance. Once the desired working distance has been reached, the set screw housed in the hole 248 is tightened in such a way that the relative distance between the stub-adapter assembly and the support member (corresponding to the desired working distance) is maintained also when the stub-adapter-support is taken out from the calibration apparatus, as illustrated in FIG. 2B.
At this point, the stub-adapter-support assembly is inserted in a pre-chamber (not illustrated) of the ion beam milling apparatus, wherein vacuum is generated. Once vacuum is generated, such assembly is brought to the main chamber (not illustrated) of the apparatus, wherein the semiconductor device is targeted by ion beams.
After the sample preparation operations are terminated, the semiconductor device may be analyzed using the electron microscope. For this purpose, the semiconductor device is inserted in a specimen holder, such as for example one of the stubs used in the target surfacing apparatus and in the ion beam milling apparatus. Such specimen holder is fitted into a pre-chamber of the electron microscope, wherein vacuum is generated. Then, the specimen holder is brought to the main chamber of the microscope, wherein the semiconductor device is analyzed by means of electron beams.
The known procedure described above requires that the semiconductor device is coupled and released to/from different specimen holders at different times. In fact, the sample preparation firstly couples the semiconductor device to the specimen holder of the target surfacing apparatus, and then to the specimen holder of the ion beam milling apparatus. Moreover, in order to analyze the semiconductor device through the electron microscope, the semiconductor device is inserted in a further specimen holder.